Hand-held power tools such as chain saws, cutting tools and grass trimmers that have internal combustion engines are known. Each of these types of machines has a working tool, such as a chain, one or more a cutting blades, or one or more cutting strings, which is brought to an operating motion by the combustion engine. Since the working tool is often close-by the operator, there is a risk of contact and an accidental injury occurring. Therefore, such machines are often equipped with various security arrangements for the protection of the operator.
The hand-held power tool may be equipped with a centrifugal clutch that engages the working tool when the engine exceeds a certain rotational speed. In normal operation, the centrifugal clutch improves safety because the working tool does not rotate, or oscillate, when the engine speed is below a clutch-in speed, at which the centrifugal clutch engages.
The hand-held power tool is normally started with the throttle valve of a carburettor of the combustion engine positioned in a starting position in order to ensure an efficient start-up. In the starting position, more air flows through the throttle valve than in an idle position of the throttle valve. Thus, the rotational speed of the combustion engine may immediately increase above the clutch-in speed when the engine starts. This may present a risk because the operator may not always be holding the hand-held tool in such a way that some of the security arrangements may provide the intended protection. Further, as the rotational speed of the combustion engine quickly rises upon starting, the clutch-in speed may be reached before the operator is ready for the working tool to begin rotating, or oscillating.
Various arrangements for avoiding unintentional rotation, or oscillation, of a working tool of a hand-held power tool at start-up of the internal combustion engine, i.e. for implementing a so-called start safety function, are known. In such arrangements a rotational speed limitation of the combustion engine is initiated to prevent the combustion engine from reaching the clutch-in speed. The rotational speed limitation has to be deactivated in order to rev up the combustion engine above the clutch-in speed. Deactivation may suitably be automatically performed when the operator is ready to use the hand-held power tool.
U.S. Pat. No. 7,699,039 discloses a method for controlling an ignition system of an internal combustion engine. A microcomputer operates a switch to control an ignition timing. The microcomputer is in communication with a speed sensor that detects the rotational speed of the engine and a speed limitation control that limits the engine speed to a limitation speed below the clutch-in speed of an included centrifugal clutch. The speed limitation control is active or activated when starting the engine. The speed limitation control is deactivated when a low speed state of the engine is detected.
Using the method of U.S. Pat. No. 7,699,039, the operator may experience some disadvantages when implementing a reliable start safety function. This disadvantage may be described as a delay to deactivate the start safety function, due to deactivation requiring the rotational speed of the combustion engine to be in the low speed state for a specified time. When the operator goes from idle to wide open throttle and back to idle again within short intervals, the rotation speed of the combustion engine does not reach the low speed state at all, or does not maintain the low speed state for the specified time. Thus, if the user intervenes in the regulating process by prematurely opening the throttle, the start safety function remains active and the user cannot increase the rotational speed above the clutch-in speed.
U.S. Pat. No. 7,735,471 discloses a method and system for controlling a light-duty combustion engine and, more particularly, a method and system that use an engine speed governor to limit the engine speed to a level that is less than a clutch-in speed of a centrifugal clutch. If it is determined that an operator is attempting to throttle or accelerate the engine, the engine speed governor is disengaged if a specific criterion is fulfilled such that normal operation can commence.
U.S. Pat. No. 8,661,951 discloses a hand-guided power tool having a drive motor and a drive element connected to and driven by the drive motor. A working tool is connected to the drive element and driven by the drive element. A braking device is correlated with the drive element, wherein the braking device in a first state, in which the braking device is applied, blocks the drive element and in a second state, in which the braking device is released, releases the drive element so that the drive elements can rotate freely. The braking device has a brake lever and a sensor that detects the first and second states of the braking device. A control unit is provided, wherein an output signal of the sensor as a state signal of the first state or the second state is supplied to the control unit. The control unit, based on the state signal, controls devices of the power tool.